Fourth Edition HEAT TRANSFER AND CHANGE OF PHASE

Transcription

1 Hewitt/Suchocki/Hewitt Conceptual Physical Science Fourth Edition Chapter 7: HEAT TRANSFER AND CHANGE OF PHASE

2 This lecture will help you understand: Conduction Convection Radiation Newton s Law of Cooling Global l Warming and the Greenhouse Effect Heat Transfer and Change of Phase Boiling Melting and Freezing Energy and Change of Phase

3 Heat Transfer Processes of thermal energy transfer: Conduction Convection Radiation

4 Conduction Conduction Transfer of internal energy by electron and molecular collisions within a substance

5 Heat Transfer: Conduction Conduction occurs predominately in solids where the molecules remain in relatively restricted locations. When you stick a nail into ice, does cold flow from the ice to your hand, or heat from your hand to the ice?

6 Conduction Insulation Doesn t prevent the flow of internal energy Slows the rate at which internal energy flows Example: Rock wool or fiberglass between walls slows the transfer of internal energy from a warm house to a cool exterior in winter, and the reverse in summer

7 Conduction Application Snow patterns on the roof of a house show areas of conduction and insulation. Bare parts show where heat from inside has conducted through the roof and melted the snow.

8 Heat Transfer: Conduction Good conductors: Composed of atoms with loose outer electrons Known as poor insulators Examples all metals to varying degrees Poor conductors: Delay the transfer of heat Known as good insulators Examples wood, wool, straw, paper, Styrofoam, cork, liquid, gases, air, or materials with trapped air

9 Conduction o Dramatic example: Author John Suchocki walks barefoot without burning his feet on red-hot coals,due to poor conduction between the coals and his feet

10 Convection Convection Transfer of heat involving only bulk motion of fluids Examples: Visible shimmer of air above a hot stove or above asphalt on a hot day Visible shimmers in water due to temperature difference

11 Cooling by expansion Convection Opposite to the warming that occurs when air is compressed Example: The cloudy region above hot steam issuing from the nozzle of a pressure cooker is cool to the touch (a combination of air expansion and mixing with cooler surrounding air). Careful, the part at the nozzle that t you can t see is steam ouch!

12 Convection Currents Convection currents produced by unequal heating of land and water. During the day, warm air above the land rises, and cooler air over the water moves in to replace it. At night, the direction of air flow is reversed.

13 Convection Reason warm air rises Warm air expands, becomes less dense, and is buoyed upward Air rises until its density equals that of the surrounding air Example: Smoke from a campfire rises and blends with the surrounding cool air.

14 Radiation Radiation Transfer of energy via electromagnetic waves that can travel through empty space

15 Heat Transfer: Radiation Wavelength of radiation is related to the frequency of vibration. Low-frequency vibrations long waves High-frequency vibrations short waves

16 Radiation Emission of radiant energy Every object above absolute zero radiates From the Sun s surface comes light, or solar radiation From the Earth s surface is terrestrial radiation in the form of infrared waves below our threshold of sight

17 Wave Frequency - Temperature (a) A low-temperature (cool) source emits primarily low-frequency, long wavelength waves. (b) A medium-temperature source emits primarily medium-frequency. (c) A high-temperature source emits primarily high-frequency, short wavelength waves.

18 Radiation Emission i of radiant energy Peak frequency of radiation is proportional to the absolute temperature of the source ( ) f ~ T

19 Emission and Absorption The surface of any material both absorbs and emits radiant energy. When a surface absorbs more energy than it emits, it is a net absorber, and temperature tends to rise. When a surface emits more energy than it absorbs, it is a net emitter, and temperature tends to fall.

20 Emission and Absorption Absorption of Radiant Energy: The ability of a material to absorb and radiate thermal energy is indicated by its color. Good absorbers and good emitters are dark in color. Poor absorbers b and poor emitters are reflective or light in color.

21 Emission and Absorption Whether a surface is a net absorber or net emitter depends on whether its temperature is above or below that of its surroundings. A surface hotter than its surroundings will be a net emitter and tends to cool. A surface colder than its surroundings will be a net absorber and tends to warm.

22 Radiation at Reflection of radiant energy Darkness is often due to reflection of light back and forth many times partially absorbing with each reflection Good reflectors are poor absorbers

23 Newton s Law of Cooling Newton s Law of Cooling Approximately proportional to the temperature difference ΔT between the object and its surroundings In short: Rate of cooling ~ ΔT Examples: Hot apple pie cools more quickly in a freezer than if left on the kitchen table Warmer house more quickly leaks thermal energy to the outside than a cooler house

24 Newton s Law of Cooling Newton s Law of Cooling (continued) Applies to rate of warming Object cooler than its surroundings warms up at a rate proportional p to ΔT Example: Frozen food warm quicker in a warm room than in a cold room

25 Global Warming and the Greenhouse Effect Greenhouse Effect Named for a similar temperature-raising effect in florists greenhouses

26 Global Warming and the Greenhouse Effect Understanding the greenhouse effect requires two concepts: All things radiate at a frequency (and therefore wavelength) that depends on the temperature of the emitting object transparency of things depends on the wavelength of radiation

27 Global Warming and the Global l Warming Greenhouse Effect Energy absorbed from the Sun Part reradiated by Earth as longer-wavelength terrestrial radiation

28 Global Warming and the Greenhouse Effect Global warming (continued) Terrestrial radiation absorbed by atmospheric gases and re-emitted as long-wavelength terrestrial radiation back to Earth Reradiated energy unable to escape, so warming of Earth occurs Long-term effects on climate are of present concern

29 Phases of Matter Matter exists in the three common phases: solid, liquid, and gas (a fourth phase of matter is plasma). When matter changes from one phase to another, energy is transferred.

30 Heat Transfer and Change of Phase Evaporation Change of phase from liquid to gas

31 Heat Transfer and Change of Phase Evaporation process Molecules in liquid move randomly at various speeds, continually colliding with one another Some molecules gain kinetic energy while others lose kinetic energy during collision Some energetic molecules escape from the liquid and become gas Average kinetic energy of the remaining molecules in the liquid decreases, resulting in cooler water

32 Evaporation Application Pigs have no sweat glands and therefore cannot cool by the evaporation of perspiration. Instead, they wallow in mud to cool themselves.

33 Heat Transfer and Change of Phase Sublimation Form of phase change directly from solid to gas Examples: dry ice (solid carbon dioxide molecules) mothballs frozen water

34 Heat Transfer and Change of Phase Condensation process Opposite of evaporation Warming process from a gas to a liquid Gas molecules near a liquid surface are attracted to the liquid They strike the surface with increased kinetic energy, becoming part of the liquid

35 Condensation Application If you re chilly outside the shower stall, step back inside and be warmed by the condensation of the excess water vapor in the shower Evaporation cools you. Condensation warms you!

36 Evaporation-Condensation Toy The toy drinking bird operates by the evaporation of ether inside its body and by the evaporation of water from the outer surface of its head. The lower body contains liquid ether, which evaporates at room temperature. (a) vaporizes, (b) creates pressure, ether goes up the tube, (c) tips over, condensed ether runs back to body. (d) each pivot wets the head and the cycle repeats.

37 Boiling Boiling process Rapid evaporation occurs beneath the surface of a liquid

38 Boiling Boiling process (continued) evaporation beneath the surface forms vapor bubbles bubbles rise to the surface if vapor pressure in the bubble is less than the surrounding pressure, then the bubbles bbl collapse hence, bubbles don t form at temperatures below boiling point when vapor pressure is insufficient

39 Boiling Heating warms the water from below. Boiling cools the water from above.

40 Pressure Cooker The tight lid of a pressure cooker holds pressurized vapor above the water surface, which inhibits boiling. In this way, the boiling point of water is greater than 100 C.

41 Equilibrium (a) In a mixture of ice and water at 0 C C, ice crystals gain and lose water molecules at the same time. The ice and water are in thermal equilibrium. (b) This gaining-and-losing process is inhibited when salt is added to the water. Then with fewer water molecules at the interface, fewer enter the ice.